A lot of these GM animals will have been modified to make them more prone to develop specific diseases such as cancer. This makes it easier to test potential cures for these diseases, particularly where a lot of animals would be needed to find a sufficient number that had the disease by chance. In these cases, using GM animals reduces the number of animals that need to be bred.

I don't think there's many research scientists who wouldn't agree that it would be better not to have to use animals at all. And people are working on alternatives – even with this increase we only use half the animals we did 30 years ago. But in the meantime there are still things we need animals for. And while we do, we should try and do what we can to make sure we use as few as possible.